The present invention relates to an automatic exposure device for use in an electrophotographic copying machine, and particularly pertains to an automatic exposure device capable of suppressing fluctuations in the luminous intensity of a lamp for irradiation of an original.
For example, in an electrophotographic copying machine equipped with a prescanning type automatic exposure device, scanning is made before copying by irradiating the original with light from an original irradiation lamp such as halogen lamp, etc., and the original's density is read by receiving the reflected light with an optical sensor. The automatic exposure device has an automatic voltage adjusting circuit for controlling the applied voltage of the original irradiation lamp so as to be constant, which assures that the intensity of light from the original irradiation lamp be kept constant.
However, there is a likehood in such an electrophotographic copying machine that a fixing heater inadvertently comes ON or OFF, while the original irradiation lamp is held in its working state. Since this fixing heater consumes a relatively large amount of electric power, as compared with other units constituting the electophotographic copying machine, it is liable to affect the automatic voltage adjusting circuit for assuring a constant voltage to the original irradiation lamp. For example, during the time that the fixing heater is held on, when the original irradiation lamp comes into its working state, the input voltage to the automatic voltage adjusting circuit falls, for example, from 100V to 92V. In response thereto, the circuit acts so as to keep its output voltage constant to apply a constant voltage to the original irradiation lamp. However, when the performance of the automatic voltage adjusting circuit is not highly accurate, it becomes difficult to keep its output voltage constant against abrupt voltage changes. Consequently, the output voltage goes down and then the applied voltage to the original irradiation lamp drops, resulting in a decrease in the luminous intensity of the original irradiation lamp. Accordingly, the automatic exposure device brings about an error in the read-out of the original's density, and consequently it becomes difficult to provide information for ensuring a proper copied image.
Also, since use of a high performance automatic voltage adjusting circuit raises the cost, it is undesirable to use such a circuit.